J. Am. Chem. SOC.1995,117, 7 1 3 9 - 7 1 5 6
7139
C-H Bond Activation by Metal Oxo Species: Chromyl Chloride Oxidations of Cyclooctane, Isobutane, and Toluene Gerald K. Cook and James M. Mayer* Contribution from the Department of Chemistry, Box 351700, University of Washington, Seattle, Washington 98195-1700 Received December 12, 1994@
Abstract: Chromyl chloride, CrO2C12, oxidizes cyclooctane, isobutane, and toluene under mild conditions (25-60 "C). The reactions give chlorinated products (chlorocyclooctane, tert-butyl chloride, and benzyl chloride) and a dark chromium-containing precipitate. Hydrolysis of the precipitate yields oxygenated products, such as ketones, aldehydes, chloro ketones, epoxides, and alcohols. Kinetic data show that all of the reactions are first order in CrO2C12 and f i s t order in substrate, with no sign of an induction period. Primary isotope effects have been observed for t-dl-isobutane and &-toluene. The kinetic and mechanistic data indicate that the reactions proceed by initial hydrogen atom transfer from the substrate to CrO2C12. The rates of hydrogen atom abstraction by CrO2C12 vary in the order cyclohexane -= cyclooctane = isobutane .c toluene and are directly related to the strength of the C-H bond being cleaved. A correlation is observed between and A F , indicating a common mechanism for the four substrates. The pathways leading from the initially formed alkyl radicals to the observed products are described. The ability of CrO2C12 to abstract a hydrogen atom from alkanes is remarkable, as it is a closed-shell, diamagnetic species, not a radical. It is proposed that the hydrogen atom abstracting ability derives from the strong 0-H bond formed on hydrogen atom transfer, in [Clz(O)Cr(OH)]. The rates of the CrO2C12 reactions correlate with rates of hydrogen atom abstraction by oxygen radicals, assuming a CrO-H bond strength of 83 kcal/mol (similar to that in HMn04-). The implications of this perspective for transition metal mediated hydrogen atom transfer reactions are discussed.
Introduction The selective oxidation of hydrocarbons is of importance in the production of fuels, commodity chemicals, and fine chemicals.' Metal oxo complexes and metal oxide surfaces are common reagents or catalysts for hydrocarbon oxidations, in the laboratory, in the industrial plant, and in metalloenzyme active sites (for instance, in cytochrome P-450 enzymes).'-4 It has been known for over a century that chromyl chloride (CrOzCh), permanganate (Mn04-), and other metal oxo compounds oxidize alkanes and arylalkane~.~.~.~ Organic radical intermedi_
_
_
~
~
Abstract published in Advance ACS Abstracts, June I , 1995. (1) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; 2nd ed.; Wiley-Interscience: New York, 1992. (b) Selective Hydrocarbon Activation, Principles and Progress; Davies, J. A,, Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; VCH: New York, 1990. (c) Activation and Functionalization of Alkanes; Hill, C. L.. Ed.; Wiley: New York, 1989. (d) Shilov, A. E. Activation of Saturated Hydrocarbons by Transition Metal Complexes; D. Reidel: Dordrecht, 1984. (e) Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: New York. 1974. Cullis, C. F.;Hucknall, D. J. Catalysis (Spec. Period. Rep.) Roy. SOC.Chem. 1982,5, 273. (f) Haber, J. Studies in Sut$ace Science and Catalysis. In Dioqgen Activation and Homogeneous Catalytic Oxidation; Simidi, L. I., Ed.; Elsevier: New York, 1991; No. 66. (2) For instance: (a) Centi, G.; Trifir6, F. Studies in Surface Science and Catalysis New Developments in Selective Oxidation; Elsevier: New York, 1990: NO. 55. (b) Solid State Chemistry in Catalysis: Grasselli, R. K., Brazdil. J. F., Eds.: American Chemical Society: Washington, DC, 1985; especially J. Haber, pp 3-21. (c) For recent studies of cobalt(1II)oxidations, see: Roelofs, M. G.; Wasserman, E.; Jensen, J. H. J. Am. Chem. SOC. 1987, 109, 4207-4217. Colussi, A. J.; Ghibaudi, E.; Yuan, 2.;Noyes, R. H. J. Am. Chem. SOC.1990, 112, 8660-8670. (3) (a) Oxidation in Organic Chemistry; Wiberg, K. B.. Ed.; Academic Press: New York, 1965; Part A. (b) Stewart, R. Oxidation Mechanisms; Benjamin: New York, 1964. (c) Oxidation in Organic Chemistry; Trahanovsky, W. S.,Ed.; Academic Press: New York, 1973; Part B. (d) Sheldon, R. A.; Kochi. J. K. Metal-Catalyzed Oxidation of Organic Compounds; Academic Press: New York, 1981. (e) Organic Syntheses by Oxidation with Metal Compounds; Mijs, W. J., de Jonge. C. R. H. I. Eds.; Plenum: New York, 1986. (f) Comprehensive Organic Synthesis; Trost, B. M.. Ed.; Pergamon: New York, 1991; Vol. 7 (Oxidation).
ates have been implicated in homogeneous, heterogeneous, and enzymatic hydrocarbon oxidations,2-8 but in general the mechanisms are not well understood and, perhaps most importantly, it is not clear what features of these oxidants enable them to oxidize C-H bonds. In many cases the details of the C-H activation step are clouded by an inability to observe and study the reactive species. We recently reported a detailed study of the oxidation of cyclohexane by chromyl chloride as a model system for C-H bond activation by high-valent metal oxo species9 CrO2Cl2 oxidizes cyclohexane by initial abstraction of a hydrogen atom
@
(4) (a) Cytochrome P-450: Structure, Mechanism, and Biochemistry; Ortiz de Montellano, P. R., Ed.: Plenum: New York, 1985. (b) Watanabe, Y.; Groves, J. T. In The Enzymes, 3rd ed.; Academic Press: New York, 1992: Vol. XX, pp 405-452. (c) FASEE J. 1992, 6, No. 2 (thematic issue devoted to the cytochromes P-450). ( 5 ) CrO?C12reviews: (a) Hartford, W. H.; Damin, M. Chem. Rev. 1958, 58, 1-61. (b) Wiberg, K. B. in ref 3a, pp 69-184. (c) Nenitzescu, C. D. Bull. SOC. Chim. Fr. 1968, 4, 1349-1357. (d) Freeman, F. Rev. React. Species Chem. React. 1973, I , 37-64. Freeman, F. in ref 3e, Chapter 2, pp 41-118. (e) See also sections of ref 3f. (6) Permanganate reviews: (a) Stewart, R. in ref 3a. pp 1-68; see also ref 3b. (b) Arndt, D. Manganese Compounds as Oxidizing Agents in Organic Chemistry; Open Court Publishing: La Salk, IL, 1981. (d) Fatiadi. A. J. Synthesis (Stuttgarr) 1987. 85- 127. (7) Formation of methyl radicals in heterogeneous methane oxidation: (a) Labinger, J. A,; Ott, K. C. J. Phys. Chem. 1987. 91, 2682-84. (b) Tong, Y.: Lunsford, J. H. J. Am. Chem. SOC.1991, 113, 4741-6 and references therein. (c) Ref 2b. (8) Carbon radical intermediates have been implicated in a number of enzymatic oxidations, including lipoxygenases,sadopamine P-hydroxylase,8b and possibly methane monooxygenases.8c (a) Boyington, J. C.: Gaffney, B. J.; Amzel. L. M. Science 1993,260. 1482-6. Glickman, M. H.; Wiseman, J. S.;Klinman, J. P. J. Am. Chem. SOC.1994, 116, 793-4 and references therein. (b) Stewart, L. C.; Klinman, J. P. Annu. Rev. Eiochem. 1988, 57, 551-592. (c) Rosenzwieg, A. C.; Frederick, C. A,: Lippard, S.J.; Nordlund. P. Nature 1993,366, 537-543. Liu, K. E.; Johnson, C. C.; Newcomb, M.; Lippard, S. J. J. Am. Chem. SOC. 1993, 115, 939-947. Priestley. N. D.: Floss, H. G.; Froland, W. A.: Lipscomb, J. D.; Williams. P. G.: Morimoto, H. J. Am. Chem. SOC. 1992, 114, 7561-7562.
0002-7863/95/1517-7139$09.00/00 1995 American Chemical Society
1140 J. Am. Chem. SOC.,Vol. 117, No. 27, 1995
Cook and Mayer
(eq 1). The resulting cyclohexyl radical is then rapidly trapped
cro2c12 +
0-
[Cb(O)CrOH]
+
(1)
by high oxidation state chromium by one of three pathways: chlorine atom transfer to give chlorocyclohexane, carbonoxygen bond formation leading to cyclohexanone, or transfer of a second hydrogen atom to give cyclohexene, which is further oxidized under the reaction conditions. The observation that a diamagnetic species such as CrO2C12 is capable of cleaving the strong C-H bond in cyclohexane is surprising. Hydrogen atom abstraction mechanisms have been viewed as the domain of radical species (with a few exceptionslO). This bias derives from our foundation in organic chemistry but has found its way into the inorganic, organometallic, and enzymatic literature as well. We suggested that radical character is not a factor in determining propensity for reaction by hydrogen atom ab~traction.~ Chromyl chloride is able to abstract a hydrogen atom primarily because of the strength of the 0-H bond formed in [C12(0)CrOH]. We proposed that the reactivity of CrO$212 is quite similar to that of an oxygen centered radical, such as ten-butylperoxy ('BuOO') and tert-butoxy ('BUD) radicals, based on the strength of the 0-H bond formed rather than any radical character. We report here studies of the oxidations of cyclooctane, isobutane, and toluene by CrO2C12. This expansion of the original study was undertaken to test the conclusions conceming both the C-H activation step as well as the subsequent steps leading to the observed products. Many studies have examined the oxidation of toluene and related alkyl aromatics by CrO2C12 (and chromic acid in various form^).^ A variety of mechanisms have been suggested, including radical (chain and nonchain) pathways, carbocation routes, and concerted mechanisms. The nature of the chromium product has also been much discussed. A charge-transfer complex is observed in reactions of CrO2C12 with a r e n e ~ , ~but ~ .the ~ ' extent of complex formation and its possible effect on the oxidation mechanism have not been addressed. Only approximate kinetic parameters and isotope effects are and a mass balance for the reaction has not been reported. In sum, a clear picture of the oxidation of toluene by CrO2C12 has not emerged. The oxidations of isobutane and cyclooctane by CrO2C12 or chromic acid have received only cursory attention.14 The studies reported here demonstrate that the CrO2C12 oxidations of cyclooctane, cyclohexane, isobutane, and toluene are very similar. All four reactions proceed via nonchain radical (9) (a) Cook, G. K.; Mayer, J. M. J. Am. Chem. SOC. 1994. 116, 18551868. (b) The calculation of D(O3MnO-H) from redox potential and pKa values has been corrected and the revised value is 83 kcaymol: J. Am. Chem. SOC.1994. 116. 8859. (IO) (a) Riichardt, C.; Gerst. M.: Nolke, M. Angew. Chem.. fnr. Ed. Engl. 1992. 31, 1523-1525. (b) Pryor, W. A. In Organic Free Radicals: Pryor, W. A.. Ed.: ACS Symposium Series 69, American Chemical Society: Washington, DC, 1978: pp 33-62. (c) Harmony, J. A. K. Methods FreeRadical Chem. 1974, 5. 101-176. (11) Hammond, P. R.: McEwan, W. S. J. Chem. SOC. (A) 1971, 38123819. (12) (a) Stairs, R. A.: Bums, J. W. Can. J. Chem. 1961, 39, 960-4. (b) Stairs. R. A. Can. J. Chem. 1962, 40. 1656-9. (c) Stairs, R. A. Can. J. Chem. 1964.42, 550-3. The kinetics of CrO?C1?oxidation of toluene and substituted toluenes were followed by preparing 10- 12 identical reaction flasks, quenching them after different reaction times, and determining the percent of reaction by iodometric titration. The, method assumes that the average oxidation state of the chromium in the Etard complex produced is constant throughout the reaction which may not be the case for toluene, as the reaction curves ([CrO?CI?] vs time) "were not of any simple form". The conclusions were based on initial rates. (13) Wheeler. 0. H. Can. J. Chem. 1964, 42, 706-707.
mechanisms, and different aspects of the radical trapping and its cascade to products are revealed by the different substrates. Most importantly, this study allows a comparison of the absolute and relative rates of hydrogen atom abstraction by CrO2C12. This confirms that CrOZC12 exhibits radical-like selectivity. The absolute rates correlate with the exo- or endothermicity of the hydrogen atom transfer step, as predicted by the Polanyi equation. Thus the rates correlate with the strength of the C-H bond being cleaved, comparing the reactions of secondary (cyclohexane, cyclooctane), tertiary (isobutane), and benzylic hydrogens (toluene). The rates also correlate with the rates of hydrogen atom abstraction from these substrates by IBuOO' and 'BuO' radicals. The implications of these correlations are discussed.
Experimental Section General. All reaction mixtures were prepared in a drybox (N? atmosphere) or by vacuum transfer of the necessary reagents. Reactions were run in greaseless Pyrex (or quartz) reaction vessels, sealed with a Teflon valve with ground glass joint attachment, and stirred with Teflon-coated stir bars. Reactions were prepared in the dark with the aid of photographic safe-lights and run under N? or in vacuo shielded from exposure to light. CrO?CI? (99.99%, Aldrich) was stored in a greaseless, light-free glass vessel and was vacuum transferred prior to use in a vacuum line greased with KRYTOX fluorinated grease (DuPont). Caution: CrOJC12 is a corrosive and carcinogenic volatile liquid that should be handled with extreme caution. Cyclohexane (99+%, Aldrich) was purifiedI5 by washing with concentrated H?S04/ m 0 3 , followed by 5% aqueous NaOH and deionized H20 until the washings were neutral. The material was then passed down a column of activated silica, predried with P205, and vacuum transferred onto sodium metal, from which it was vacuum transferred before use. Cyclooctane (Columbian Carbon Co.) was purified15 by washing with concentrated HzS04 followed by saturated aqueous NaHCO3 and deionized H?O until the washings were neutral. The material was then dried over MgS04 and filtered, and the resulting filtrate was fractionally distilled and the middle fraction collected. After passing this material down a column of activated silica it was further dried over sodium metal then distilled under Nl. The liquid collected was degassed on the vacuum line before storage in a greaseless, Teflon-valve-sealed container in a drybox. Isobutane (Aldrich. 99%) and isobutylene (Aldrich, 99+%) were used as received. tert-Butyl alcohol (Baker, 99%) and isopropyl alcohol (Aldrich, 99.9%) were dried over sieves and vacuum transferred prior to use. Toluene (Baker, 99.9%) was purifiedL5by washing with concentrated HlS04 while being cooled in an ice bath. This was followed by washing with saturated aqueous NaHCO3 then deionized H?O until the washings were neutral. The material was then pre-dried with P?o5 and vacuum transferred onto sodium metal, from which it was vacuum transferred before use. dgToluene (Cambridge Isotope) was dried over sieves, stored over CaH?, and vacuum transferred before use. tert-Butylbenzene (Aldrich, 99%) was p ~ r i f i e din ' ~the same manner as toluene except after washing with cold concentrated H?S04 the material was washed repeatedly with 4% aqueous NaOH and then deionized H?O until neutral. The material was then dried over MgS04 and filtered, and the resulting filtrate was further dried over sodium metal from which it was fractionally distilled under N1 and the middle fraction collected. This liquid was degassed (14) (a) CrO?CI? oxidation of 2-methylbutane to 3-methyl-2-butanone and 3-chloro-3-methyl-2-butanone: Hobbs, C. C.. Jr.; Houston, B. J. Am. Chem. SOC. 1954. 76. 1254-1257. (b) Oxidations of isobutane and cyclooctane by CrO3 in acetic acid: Mareg, R.: RoEek, J. Collect. Czech. Chem. Commun. 1961. 26. 2370-2388. Mares. R.; RoEek, J.: Sicher, J. Collect. Czech. Chem. Commun. 2355-2368. Rates of oxidation are reported. as is the formation of cyclooctanone from cyclooctane. (c) Oxidation of triethylmethane to triethylcarbinol by Na2Cr20.i in acidified aqueous acetic acid: Sager, W. F.: Bradley, A. J. Am. Chem. SOC. 1956, 78, 1187- 1190. (d) Oxidation of (+)-3-methylheptane to (+)-3-methylheptanol by Na2CrZO.l in perchlonc acid: Wiberg, K. B.;Foster, G. J. Am. Chem. SOC.1961. 83, 424. (15) Perrin, D. D.: Armarego, W. L. F. furifcarion of Luboratoy Chemicais. 3rd ed.: Pergamon: New York, 1988.
C-H Bond Activation by Metal A
F?
Oxo Species
B
Figure 1. Drawings of the long-path UV-vis cell and cell holder used in the kinetic experiments: (A) exterior view: (B) cut-away view of cell and cell holder, showing the well for the reaction solution at the bottom of the cell and the Teflon stopcock and 24/40 glass joint at the top. The air-driven stir motor is not shown. The apparatus is mounted in the spectrometer such that the light path proceeds through the vapor above the reaction solution.
on the vacuum line before storage in a greaseless, Teflon-valve-sealed container in a drybox. All the other reagents were used as received. Organic oxidation products were identified and quantified by application of a variety of techniques depending on the substrate (see below). GC/FIDanalyses were performed on a Hewlett Packard 5790A instrument connected to a Hewlett Packard 3390A integrator. Products were identified by comparison of their GC retention times with authentic samples and by G C M S using a Kratos Profile mass spectrometer. Product quantification by GC/FID was done by one-point calibration to an internal standard. 'H NMR spectra were obtained using a Bruker AM-300 spectrometer. Chemical shifts are referenced to residual protons in the solvent or (in D?O) to DSS (Me3SiCH?CH?CH?SO3Na, sodium 2,2-dimethyl-2-silapentane-5-sulfonate, MSD Canada), which was also used as an internal standard for quantitation of the products. IR analyses for CO? were performed using an 8 cm quartz gas IR cell in a Perkin-Elmer 1720 FT-IR flushed with N?. HPLC analyses were accomplished with a Hitachi L-6200 pump connected to a Hitachi L-4250 UVlvis detector set at 250 nm and a Hitachi D-2500 integrator. HPLC separation was accomplished using a Beckman 25 c d 4 . 6 mm Ultrasphere ODS column running 80% Millipore H2O (0.1% TFA) and 20% acetonitrile. Products were identified and quantified by comparison of their HPLC retention times and intensities with authentic samples using potassium hydrogen phthalate (KHP) as an internal standard. Kinetics. Reactions were followed by UV/vis analysis of the CrOlC12 in the vapor phase above the reaction solution. This general method for kinetic data collection has been described for the reaction of CrO2C12 with c y c l ~ h e x a n e . ~Cyclooctane. ~ isobutane, and toluene react at lower temperatures than cyclohexane, so longer pathlength cells were used to make up for the lower vapor pressure of CrO?Cl? above the reaction solutions. An 8 cm path length quartz cell (total volume = 21.6 mL) sealed to a Teflon needle valve was used for the cyclooctane reactions, and a similar thick-walled Pyrex cell (8 cm path length, total volume = 17.1 mL) was used for toluene and isobutane reactions (the latter were run at elevated pressures, 3-4 atm). Reaction temperatures were regulated by complete immersion of the cell volume and headspace in a water-tight chamber with quartz windows (Figure 1) connected to a circulating water bath (Lauda Model K-2R). Complete immersion is required to prevent refluxing within the cell. Rapid equilibration between the vapor and liquid phases is facilitated by continuous stimng of the reaction solution using an air-driven stimng plate mounted in the base of the cell holder. This apparatus, like the original? is mounted in the sample compartment of a Hewlett Packard 8452A diode array spectrophotometer. The CrO?Cl? absorbance in the vapor at A,, = 292 and 408 nm was shown to be linearly related to the CrO?Cl?
J. Am. Chem. SOC., Vol. 117, No. 27, 1995 7141 concentration in cyclohexane solution at 25 "C. A spectrum of the vapor of the appropriate solvent at the fixed temperature was used as the blank. Analysis of the kinetics data was performed as described previously for reactions of cyclohexane with CrO?C12.9a The use of the 8 cm path length cells greatly reduced the contribution to the apparent absorbance due to precipitate (Etard complex) forming in the light path. For reactions employing the Pyrex cell, which cuts off the peak with A, = 292 nm, the CrO?Cl? concentration at time t, [CrO?C12],, was calculated using the CrO?C12 A- at 408 nm in conjunction with a single reference wavelength, 352 nm. Iodometric Titrations of Etard Complexes. Iodometric titrations were performed under a flow of N?, following the procedures of Vogel,16 using solubl: starch as the indicator. The same procedure was used on isolated Etard complexes derived from cyclooctane, isobutane, and toluene; a sample procedure follows. After heating a reaction mixture of 0.123 mmol of CrO?Cl? in 1.00 mL of cyclooctane for 5 h at 40 "C, the volatiles were removed in vacuo. To the brown solid remaining in the reaction vessel was added 4 mL of 0.92 M aqueous KI, followed immediately by 2 mL of 0.35 M aqueous HzS04 and 5 mL of deionized H20, giving a clear brown/green solution. A buret containing 25.5(3) mM aqueous Na2S203 solution (concentration determined by titration of K2Cr207 primary standard done the same day) was connected to the reaction vessel via a rubber septum. After addition of 2 mL of the Na2S?03solution from the buret to the solution in the reaction vessel, the buret was momentarily disconnected as 3 drops of a near-saturated solution of aqueous soluble starch (prepared shortly before the titration, by addition of soluble starch to boiling H2O) was added to the solution in the reaction vessel. This resulted in a color change to clear navy blue. The buret was reconnected to the reaction vessel and the titration was continued to the end point, a rapid color change from navy blue to pale green, upon addition of 4.10 f 0.07 mL of Na2S203 solution. A repeat of this procedure on a second sample of complex resulting from an identical reaction reqyired 4.25 f 0.07 mL. It has been shown previously, for titrations of Etard complex resulting from reaction of cyclohexane, that varying the concentration of the KI solution by a factor of 15 has no significant effect on the results, indicating that chromium is efficiently reduced by I- rather than by the organic matter presents9 Reactions of Cr02C12 with Cyclooctane. In a typical procedure, 1.00 f 0.02 mL of cyclooctane was measured out using a volumetric flask and added to a reaction vessel containing a stir bar. Into the flask was syringed 10.0 f0.2 p L of CrOZCl? (0.123 "01). The needle valve on the reaction flask was quickly closed and the vessel was removed from the glovebox. The flask was then stirred for 5 h in a light-free, temperature-controlled circulating water bath at 40 "C, over which time a brown precipitate formed and the solution turned from clear red to colorless. The flask was removed from the water bath and opened to the atmosphere, and 1 mL of 0.2 M aqueous NalS203 was added to give a clear green aqueous layer and a colorless organic phase. After addition of 5 mL of diethyl ether and 10.0 f 0.2 p L of tertbutylbenzene standard, the resulting mixture was allowed to stir for several minutes. An aliquot of the organic layer was then removed and analyzed by GC/FID and GCMS. Yields are given in Table 1. For reactions of CrO?C12 with cyclooctane diluted with cyclohexane, a stock cyclooctanekyclohexane solution was first prepared in the glovebox from 2.457 f 0.001 g of cyclohexane (0.02919 mol) and 3.898 f 0.001 g of cyclooctane (0.03474 mol) for a total volume of 7.88 & 0.05 mL. Reactions mixtures were prepared and worked up as described above. At 40 "C reactions of 10.0 f 0.2 p L of CrO?Cll (0.123 mmol) in 1.00 f 0.02 mL of the cyclooctanekyclohexane stock solution proceed to completion in 9 h. Syntheses of Oxidation Products. Chlorocyclooctane was synthesized from cyclooctene by hydrochl~rinationl~ and isolated as a clear liquid after chromatography on silica gel with pentane. The product was 97% pure by GC/FID. GC/MS: M+ = 146. 'H NMR ((336): 6 (16) Jefferey, G. H.: Bassett, J.: Mendham, J.: Denney, R. C. In Vogel's Textbook of Quantitative Chemical Analysis. 5th ed.: Wiley: New York, 1989; pp 384-393. (17) Kropp, P. J.; Daus, K. A,; Tubergen, M. W.; Kepler, K. D.; Wilson, V. P.; Craig, S. L.; Baillargeon. M. M.: Breton, G. W. J. Am. Chem. SOC. 1993, 115. 307 1-3079.
7142 J. Am. Chem. SOC.,Vol. 117, No. 27, 1995
Cook and Mayer
Table 1. Product Yields from Reactions of CrO?C1? and Cyclooctane
ntx 25OC (7.42M) neat 40°C (7.31M)
14.5(2)
190)
3.0(1)
3.4(1)
2.6(2) 3.94(3)
68(2)%
6.6%
2.04(6)
13.9(5)
19.6(6) 3.6(2)
3.6(2)
2.7(1)
3.87(3)
68(2)%
6.7%
2.00(6)
0.123M
+ ( 3 4 1 2 40°C
12.8(5)
19.9(2) 4.0(2)
1.5(2)
2.8(6)
3.88(3)
67(2)%
6.9%
2.09(6)
0.123M
(4.35M) neat 58'C (7.18M)
14.33)
19.6(4) 4.4(1)
3.0(2)
2.3(2)
3.74(3)
66(2)%
7.7%
1.94(6)
0.123M 0.123M
Undetected oxidation oroducts
0 . 3 0 8 m o l cr42c12, 40.c 0.0384 "01 c8H14. in 1.25 mL QH12 0 . 2 4 7 m m O l ~ C l 2 25 'C 0.0384 TIOI ~ C a 1 4 in 1.60 mL (3412
---
% yields versus cyclocctene reactedc tlam 34(2) -20(3)
UWC
29(3)
--
19(4)
46(4)
520)
'' Initial concentration of CrO?CI?. Molar concentration of neat cyclooctane calculated from its density, from ref 20. The concentration of cyclooctane in cyclohexane assuming ideal behavior in both liquid and vapor phases. Yields averaged from at least two reactions; the value in parentheses is one standard deviation. Average chromium oxidation state in the isolated precipitate (Etard complex) determined by iodometric titration. Value reported is the average from two experiments. Yield calculated from oxidative equivalents unaccounted for in the observed products. The average number of moles of CrO?CI? consumed per mole of cyclooctane product formed (the reaction stoichiometry). 3.93 (m, 1 H), 1.85 (m, 4 H), 1.1-1.7 (m, IO H). 2-Chlorocyclooctanone: In the glovebox, a solution of 0.5 g (4.5 mmol) of cyclooctene in 5 mL of cyclohexane was added dropwise to a stirred solution of 1.4 g (9 mmol) of CrOzC12 in 5 mL of cyclohexane over the course of IO min. A brown precipitate formed immediately. This mixture was heated to 50 "C for 3 h before the reaction was quenched (in the air) with 10 mL of 0.1 M aqueous Na&O3 solution. After extraction with Et?O, 2-chlorocyclooctanone was isolated by column chromatography on silica gel, eluting with pentane then with 5% (v/v) EtOAc in pentane. Further purification with a second column, using 2% EtOAc in pentane, gave ca. LOO mg of a clear, colorless, liquid that was 84(1)% 2-chlorocyclooctanone by GCFID, with a 10( l)% cyclooctanone impurity. GCIMS: M+ = 160. 'H NMR ( c a b ) : 6 3.95 (dd, 6 Hz, 4 Hz, 1 H), 2.35 (m, 1 H), 1.6-2.1 (m, 3 H), 0.8-1.6 (m, 8 H). Cyclooctanol was formed virtually quantitatively by reduction of cyclooctanone by N a B h in methanol,lS '98% pure by GCFID. GC/ MS: M+ = 128. 'HNMR (C6D6): b 3.62 (m, 1 H), 1.1-1.9 (m, 15 H). Cyclooctene oxide: m-Chloroperbenzoic acid oxidation of cyclooctene in CH2C12, following standard procedures,19 gave material that was 91( I ) % cyclooctene oxide and 6( I)% m-chlorobenzoic acid by GC/FID. GC/MS: M+ = 126. 'H NMR (C6D6): 6 2.61 (m, 2 H), 1.94 (m, 2 H), 1.0-1.4 (m, 10 H). Reaction of Cr02C12 with Cyclooctene. A reaction vessel was charged with 0.308 mmol of CrO?C12, 1.00 mL of cyclohexane, a stir bar, and a sealed breakseal vial containing 0.038 mmol of cyclooctene in 250 p L of cyclohexane, which had been sealed under vacuum. After 10 min of stirring in a 40 "C water bath, the reaction flask was quickly shaken, breaking the vial containing the cyclooctene solution, and returned to the bath. Every 30 min the reaction flask was periodically removed from the bath and vigorously shaken to ensure complete mixing. After 2 h, a significant quantity of brown precipitate was visible in the bottom of the flask but the supernatant remained clear red in color suggesting that some of the CrO?Cl? remained unreacted. The reaction was quenched by the addition of I mL of I M aqueous
Na&03, followed by 5 mL of Et20 and 5.0 i 0.2 p L of tertbutylbenzene. The Et20 layer was analyzed by GC/FID (Table 1). A similar reaction of CrO?C12 with cyclooctene at 25 "C was accomplished in the glovebox by adding 50-pL aliquots of a solution of 0.038 mmol of cyclooctene in 300 p L of cyclohexane, every 5- IO min, to a solution of 0.247 mmol of Cr02C12 in 1.00 mL of cyclohexane that had been initially stirred for 40 min. After six aliquots, the cyclooctene vial was rinsed with 300 pL of cyclohexane, these washings were added, and the solution was stirred for 20 min prior to workup as above. Kinetics of Cr02C12 Oxidation of Cyclooctane. Solutions were prepared in the glovebox by dilution of the appropriate volume of CrO2Cl? to 1.00 i 0.02 mL with either cyclooctane or cyclooctane/ cyclohexane solution. The solution and a Teflon covered stir bar were added to the 8 cm quartz cell. The cell was evacuated on the vacuum line, after the reaction solution had been cooled to -78 "C. Reaction solutions were allowed to thaw prior to placing the cuvette in the special cell holder, as described above. Spectra were taken from 250 to 600 nm. The data analysis was performed as described above. The cyclooctane concentrations of neat cyclooctane were calculated from the molar densities at the appropriate temperatures.?O Concentrations in cyclooctane/cyclohexane mixtures were calculated using the molar densities of the pure liquids, assuming ideal behavior. Reaction conditions and rate constants are reported in Table 2. Reactions of Cr02C12 with Isobutane. Reaction vessels with consistent internal volumes (with stir bar, 26.8(7) mL) were constructed to ensure reproducible isobutane concentrations. In the glovebox, a reaction vessel was charged with 1.00 i 0.02 mL of cyclohexane, a stir bar, and 10.0 & 0.2 ,uL of CrO?Cl?(0.123 mmol). The vessel was attached to the vacuum line, cooled to -78 "C, and evacuated. The vacuum manifold (internal volume = 1.078 L) was filled to 130 Torr with isobutane (7.6 1 mmol, assuming ideal behavior). The isobutane was condensed into the reaction vessel cooled in liquid N?. The reaction vessel was stirred for 18 h in a light-free, temperature-controlled water bath at 60 "C, forming a brown precipitate while the solution turned
( 18) Reduction Techniques and Applications in Organic Synthesis: Augustine, R. C., Ed.; Marcel Dekker: New York, 1968; pp 21-34. (19)Carlson. R. G.; Behn, N. S. J. Org. Chem. 1967, 32, 1363-1367.
(20) Design Institute for Physical Property Data (DIPPR) file, American Institute of Chemical Engineers (AIChE), provided by the Scientijk &
Technical Information Network (STN).
J. Am. Chem. Soc., Vol. 117, No. 27, 1995 7143
C-H Bond Activation by Metal Oxo Species Table 2.
Rate Constants for Reactions of CrO?C12 with Cyclooctane, Isobutane, and Toluene ~~
reaction
tempa "C
initial [CrO2C12], mom
substrate concn,b mom
kot,s.cdx
sdi
ke x 10-5
M-1
s-I
Cyclooctane f 1 2a 2b 3a 3b 4
25.00 32.97 32.97 40.00 40.00 40.00
0.123 0.123 0.123 0.123 0.123 0.123
7.42 7.36 7.36 7.31 7.3 1 4.35
5a 5b 6a 6b
50.00 50.00 57.80 57.80
0.0987 0.0987 0.0617 0.0617
7.24 7.24 7.18 7.18
8.7Qf0.39) 9.02(*0.19) 19.20(&0.06) 18.36(&0.13)
12.1(10.5) 12.5(f0.3) 26.7(&0.1) 25.6(10.2)
40.00 40.05 50.00 50.10 60.00 60.00 68.50 68.50 59.90 80.00
Isobutaneh 0.07 19 0.0652 0.0630 0.0834 0.0763 0.0753 0.037 1 0.0374 0.0736 4.8 x 10-5
3.70 4.21 4.21 2.67 3.05 3.12 3.08 3.03 3.25 0.161
1.15 1.31 4.18 2.58 8.12 7.19 16.30 15.82 4.61 0.69
0.311 0.312 0.992 0.967 2.66 2.30 5.29 5.22 I .42 43
29.40 29.36 40.12 40.12 40.14 49.93 49.93 59.90 59.90 50.00 50.00
0.122 0.122 0.0843 0.0843 0.0603 0.0596 0.0596 0.0588 0.0588 0.0596 0.0596
1.85 1.85 1.37 1.83 1.19 1.17 1.17 1.16 1.16 1.17 1.17
3.53 3.45 5.96 7.70 4.95 11.56 11.25 25.32 25.14 1.48 1.44
60.00
rerr-Butylbenzene 0.123
6.46
7a 7b 8a 8b 9a 9b 1Oa 10b 11 12
+ + + + +
Cfl?Cl? CjHio CrO?CI? + CjHio Cfl2Cl? CjHio Cfl?C12 CjHio CrO?Cl? CjHio CjHio CQCl? CrO?Cl? C ~ H I O CrO?Cl? CjHio Cfl2Cl? CjHgD' Cfl?Cl? CjHio gas phase reaction
+ +
+ +
0.729(*0.006) 1.67(*0.03) 1.63(*0.07) 3.3 l(10.02) 3.38(&0.07) 1.77(&0.01)
0.983( *0.08) 2.27(10.04) 2.2 l(f0.09) 4.53(10.03) 4.62(*0.10) 4.07(10.03)
Tolueneh 13a 13b 14a 14b 14C 15a 15b 16a 16b 17a 17b 18
+
CrO?Cl? PhCMe3
19.1 18.7 43.7 42.1 41.7 98.7 96.1 219 217 12.6 12.3
0.407
0.63
Temperature fO.10 "C. The calculation of substrate concentrations is presented in the Experimental Section. The errors reported for cyclooctane are the spread of the individual data treatments (see Experimental Section). For the other substrates, the data treatment does not generate an estimate of error; errors are estimated at f 3 % . The rate constants reported for reactions of isobutane are based on initial rates and have been adjusted to account for competitive attack at the cyclohexane solvent. Second-order rate constants calculated by dividing kobs by the concentration of-substrate at the stated temperature. f Reactions-of cyclooctane were! run in neat substrate unless otherwise noted. Reaction run in the uresence of 46% by mole cyclohexane. Reactions run in cyclohexane, except for the gas-phase reaction. Deuterated at the tertiary position: (C'H&CD. ti
colorless. These steps were all performed in the dark with the aid of a photographic safe-light. The volatiles were collected by short-path vacuum transfer directly from the reaction vessel. Toluene (0.7 mL) and methylcyclohexane standard (5.0 rt 0.2 pL) were added to the isolated volatiles, which were analyzed by GCFID and G C / M S . CD3CN ( 0 5 mL) was vacuum transferred into the reaction vessel containing the Etard complex and, after stirring for several minutes, the volatiles were collected by shortpath vacuum transfer into a flask containing 1.81 pmol of DSS standard. To the solid remaining in the reaction flask, 0.5 mL of 1 M Na2S203 in D?Owas added by syringe, under flow of N?. After degassing the reaction flask, the volatiles were collected by short-path vacuum transfer to the flask already contaging the CD3CN solution. In this manner, organic products in the Etard complex such as acetone, rea-butyl alcohol, and cyclohexanone were quantitatively recovered, as verified by control experiments. Initial treatment with CD3CN before hydrolysis is necessary to avoid oxidation of aldehyde products during aqueous NaZSZO3 workup. This could be avoided by employing the more efficient Cr(V1) reductant, KI; however, the 12 that vacuum transfers with the products created other problems. The CD3CN/D?O solution collected was analyzed by 'H NMR. Products were identified and quantified by 'H NMR spectra and by comparison with authentic samples (see below) whenever possible (Table 4). The reaction mixtures and authentic samples were stable in the CD3CN/D?O solution. Reactions of CrO?Cl? with t-dr-isobutane, ( C H M D , were performed and analyzed in the same manner as described above, with longer reaction times because of the mild isotope effect (Table 4). r-d,-
Isobutane was prepared as follows. A solution of 23 mL of 2 M 'BuMgC1 in Et20 (Aldrich) in 50 mL of dry triglyme was left under dynamic vacuum for 3 h to remove all traces of the Et20 solvent. D?O (0.39 g, Cambridge Isotope, 99.9%) was vacuum transferred into the reaction vessel cooled in liquid N2. The reaction was stirred at 25 "C for 15 h, then cooled to 0 "C, and the r-dl-isobutane was vacuum transferred into a flask at -196 "C. Yield: 1.16 g, 43% based on 'BUMgC1. GC/MS: 59. 'H NMR (CbDb): 6 0.84 (1:l:l t, 3 J =~ 1 ~ Hz). No tertiary proton resonance is observed, so the enrichment is estimated to be '98%. Syntheses of Oxidation Products. 2-Chloro-2-methylpropionaldehyde was prepared following the procedure in ref 2 1. GCMS: 106. IH NMR (D?O): 6 4.91 (s, 1 H), 1.54 (s, 6 H). 2-Hydroxy-2methylpropionaldehyde was produc d in D?O solution by HzS.04catalyzed hydrolysis of its dimethylac tal.?l 'H NMR (DzO): 4.76 (s, 1 H), 1.17 (s, 6H). If-Hydroxy-2-methylpropane was generated in situ by addition of 0.5 equiv of KOH to a solution of isobutylene oxide in DrO." 'H NMR (D?O):3.40 (s, 2H), 1.18 (s, 6H). Attempts to were not successful. Sinsynthesize 2-chl0ro-2-methyl-l-propanol~~ glets at 6 = 3.52 and 1.52 in the IH NMR (CD3CN/D?O) of the isobutane reaction products are assigned to this compound because (i) they were consistently in a 1:3 ratio, (ii) the upfield chemical shift is
J
(21)Stevens, C. L.: Gillis, B. T. J. Am. Chem. Soc. 1954, 76, 3448345 1. (22) Parker, R. E.: Isaacs, N. S. Chem. Rev. 1959, 59, 737-799. (23)Stewart. C. A,: VanderWerf, C. A. J. Am. Chem. Soc. 1954, 76, 1259- 1264.
7144 J. Am. Chem. Soc., Vol. 117, No. 27, 1995
Cook and Mayer
Table 3. Rates, Isotope Effects, and Activation Parameters for the Initial Step of Reactions of CrOZC12 with Cyclohexane, Cyclooctane, Isobutane, and Toluene
Data for cyclohexane from ref 9a. Rate constant per reactive hydrogen (12 per molecule of cyclohexane, 16 for cyclooctane, 1 for isobutane, and 3 for toluene). At 340 K. Values in { } are activation parameters on a per hydrogen basis. See text. e kC6H,JkC6D12 measured at 75.00 "C. fDetermined from the yields of (CH3)3CCI and C ~ H I I in C ~reactions of (CH3)3CH and (CH&CD in cyclohexane at 59.90 "C. s kC6H5CH3/kC6DSCD3 measured at 50.00 "C. Table 4. Product Yields from Reactions of CrO2C12 with Isobutane and Related Substrates
naction 1. CrO2C12 (0.0762M)+
2.
3a.
b.
4.
5.
MeCH (3.07M)in cyclohexane (5.69M) CrO2Cl2 (0.0736M)+ Me3CD(3.25M)in cyclohexane (5.51 M) CrOzC12 (0.247mmol)+ MQC=CH~(52poI) in cyclohexane(1.oomL) CrOf212 (0.247mmol)+ Me2C=CH2 ( 5 4 ~ 0 1in) cyclohexane(1.oomL) CrO2Clz (0.247mmol)+ MqCOH (-01) in cyclohexane (1.00mL) CrOfll2 (0.185mmol)+ MezCHOH (65pol) in cyclohexane (1.oomL)
60°C
60'C
7.aC7) 3.9(2) 5.7W 0.70(6) 0.432) 0.28(2) 0.40(9) 0.20
2.0(3)% 3.49(3) chlorocyclohexanec 2.7%chlorocyclohexane'
0.2
0.3
0.3
0.3
0.3
d
*-
12.5 27.4 3.9
3.3
--
2.5
0.5
0.8
d
d
--
9.3
37.7 5.0
10.3
--
2.9
0.5
1.7
d
d
60'CB 4 h
--
5.3
2.4
0.7
1.4
6.1
0.9
0.1
0.5
d
d
2SoC, 1 h; then 60°C. 2h
--
100
--
--
60°C. lo min
3.5
4.5
0.61
0.7
6O0C, lh
4.5
0.13
_-
Yields reported versus moles of the limiting reagent, CrO?CI? in entries 1 and 2 and the organic reagent in entries 3-5. Errors reported from the average of two or more experiments. Average chromium oxidation state in the isolated precipitate (Etard complex) determined by iodometric titration. Value reported is the average from two experiments. Cyclohexanone and 2-chlorocyclohexanone also observed but could not be quantified separately by 'H NMR. Total yield of the two products = 2.6(2)%. dProducts were not quantified. 'Cyclohexanone and 2chlorocyclohexanone were also observed but could not be quantified separately by 'H NMR. Total yield of the two products = 3%. [I
within 0.05 ppm of that of 2-chloro-2-methylpropionaldehydeand 20.3 ppm downfield from the tertiary alcohol products, and (iii) they were only observed upon hydrolysis of the Etard complex, not in the CD3CN volatiles prior to hydrolysis, suggesting that the product is an alcohol. Determination of CO2 Yield. The volatiles from a standard reaction were vacuum transferred into the 8 cm path length quartz cell. The cell was mounted in the chamber of the IR spectrometer under a nitrogen flow and vigorous purging was continued until absorbance at 2360 cm-' became constant. Spectra were taken of the headspace above the cyclohexane/isobutane solution using the evacuated cell under N? purge as the blank. Absorbance due to isobutane in the headspace was subtracted independently using a stored spectrum. Spectra showed the presence of CO? (Y,,, = 2360 cm-l) but not CO. CO?was quantified (0.34(2)% versus CrO?C12) by addition of a known quantity of CO2 from a gas addition bulb to the cell still containing the reaction volatiles and determination of an extinction coefficient by comparison of the change in the CO? absorbance with the moles added. To determine the amount of carbonate or bicarbonate, the Etard complex from a standard reaction was treated with 164 mg of solid Na2.9203 5Hz0 under flow of N?, the reaction vessel was evacuated, and 0.5 mL of degassed 0.1 M aqueous HCI solution was added by (24) From a third-order polynomial fit to the isobutane vapor pressure vs temperature data in: Stull, D. R. fnd. Eng. Chem. 1947, 39, 517-550.
vacuum transfer. After the solution was stirred for several minutes, the volatiles were vacuum transferred into the 8-cm path length quartz cell and analyzed for CO? as above, giving a yield of 0.25(2)%. Using the above procedures on a sample of K2CO3 gave quantitative recovery of carbonate as CO?. Reactions of CrO2C12 with Isobutylene. In the glovebox, a reaction vessel was charged with 0.247 mmol of Cr02C12, 1.00 mL of cyclohexane, a stir bar, and a sealed breakseal vial containing 52.4 pmol of isobutylene gas. The reaction vessel was degassed on the vacuum line and stirred for 1 h at 60 OC and then quickly shaken, breaking the isobutylene vial. After an additional hour of stimng at 60 "C, a brown solid had formed in the bottom of the reaction vessel but the supematant remained red, suggesting that not all of the CrOzClz had reacted. Workup was as described above for isobutane reactions. A similar reaction was done with stirring for only 10 min after the breaking of the vial. Yields are reported in Table 4. Reaction of CrO2C12 with tert-Butyl Alcohol and Isopropyl Alcohol. In the glovebox, 0.247 mmol of CrO?C12 was added to a standard reaction vessel containing 63.6 pmol rerr-butyl alcohol in 1.OO mL of cyclohexane and a stir bar. The reaction vessel was attached to the vacuum line and evacuated while being cooled to -78 OC. The reaction was stirred at 60 "C for 4 h and worked up as described for isobutane. A similar reaction of 0.185 mmol of CrO?C12 and 65.3 pmol of isopropyl alcohol was stirred at 25 "C for 1 h and at 60 OC for 2 h.
C-H Bond Activation by Metal
J. Am. Chem. SOC., Vol. 117, No. 27, I995 7145
Oxo Species
Table 5. Product Yields from Reactions of CrO?C12 with Toluene and Toluene-ds
0.103M 0.060M
1.18M
5OoC
18(1)
32.0(7)
5(1)
1.18M 50°C &toluene
21(2)
28(2)
4.4(1)
3.92(3)
84(3)% 78(4)% e
5.6% 7.5%
1.636) 1.64(6) e
Yields averaged from at least two reactions and reported versus moles of CrO2C12 reacted. Average chromium oxidation state in the isolated precipitate (Etard complex) determined by iodometric titration. Value reported is the average from two experiments. Yield calculated from oxidative equivalents unaccounted for in the observed products. Represents stoichiometry of reaction in terms of average moles of CrO?CI? consumed per mole of product formed. e Value calculated assuming the same average chromium oxidation state observed for the protio reaction (3.92). Procedures for workup and analysis were as described for reactions of isobutylene; yields are reported in Table 4. Kinetics of CrO2C12 Oxidation of Isobutane. Reaction solutions were prepared in an 8 cm path length thick-walled Pyrex cell (to withstand the isobutane pressure), in the same manner as described above for reactions of cyclooctane. Data were collected and analyzed using the same techniques and apparatus employed to follow the kinetics of cyclooctane oxidation. Scans were taken over the wavelength range from 300 to 700 nm using the cyclohexane solvent as a blank (vapor phase, at the appropriate temperature). The solution-phase isobutane concentrations for the various reactions were calculated from eq 2, derived using Raoult's law and the Ideal Gas law, assuming ideal behavior for both isobutane and cyclohexane in both the liquid and gas phases (including that molar densities of pure cyclohexane and isobutane are accurate for the mixed solvent). In eq 2, n = moles of isobutane in the vapor, PO = vapor pressure of
pure isobutane at the appropriate temperat~re,?~ x = moles of isobutane added, d, = molar density of pure isobutane at the appropriate temperature,'O R = gas constant, T = temperature, c = moles of cyclohexane added, d, = molar density of pure cyclohexane at the appropriate temperature,?O and V = volume of the empty cell (or reaction vessel). The expression accounts for the decrease in headspace volume due to the increase in solution volume from isobutane dissolving in the cyclohexane solvent. However, it does not account for partitioning between liquid and gas of the cyclohexane, which should represent a very modest perturbation. Observed rate constants and second order rate constants calculated from the isobutane concentrations are reported in Table 2. Reactions of C r 0 & with Toluene. In the glovebox, a reaction vessel was charged with 1.00 f 0.02 mL of cyclohexane, a stir bar, 150 f 1 p L of toluene (1.41 mmol), and finally 10.0 f 0.2 p L of CrOzC12 (0.123 mmol). The reaction vessel was removed from the glovebox, cooled to -78 "C, and evacuated on the vacuum line. The flask was stirred at 50 "C for 1.5 h, over which time a brown precipitate formed while the solution tumed colorless. The volatiles were collected by short-path vacuum transfer while the reaction vessel was gently heated to 40-50 "C for 5 min. Cyclohexane (1.0 mL) and rerrbutylbenzene standard (10.0 f 0.2 pL) were added to the isolated volatiles and the solution was analyzed by GC/FID, showing benzyl chloride as the only volatile product. The isolated Etard complex was dissolved in 2 mL of 0.9 M aqueous KI, followed by 2 mL of CH3CN. and diluted to 10.00 f 0.08 mL with deionized HzO washings from the reaction vessel. A 1.00 & 0.01 mL aliquot of this solution was added to a 1.OO f 0.01 mL aliquot of a 0.05051 M aqueous potassium hydrogen phthalate solution (KHP) as an intemal standard and analyzed
by HPLC, showing benzaldehyde and benzyl alcohol. Reactions of ds-toluene were done in the same manner. HPLC detector responses to the deuterated products were assumed to be the same as for the corresponding protiotoluene oxidation product. Yields are given in Table 5 . Detection of Benzoate from Toluene Oxidation. Zn/HCI reductions of isolated Etard complex were performed following standard procedures.?: Under flow of N?, 2 mL of 0.9 M aqueous KI was added to isolated Etard complex from reaction of 0.123 mmol of CrO?Cl2 with 1.41 mmol of toluene in 1.00 mL of cyclohexane at 50 "C. After allowing the resulting solution to stir for several minutes the volatiles were removed in vacuo while the reaction vessel was warmed to 4050 "C. Under flow of N?, 600 mg of Zn metal was added to the brown solid remaining in the reaction vessel and 5 mL of 9 M aqueous HCI was added dropwise to the reaction vessel by cannula, resulting in Hz evolution and a slow color change from clear emerald green to clear robin's egg blue. Another 100 mg of Zn metal was added under flow of N?, followed by 4 mL of CH3CN by cannula. Once all of the Zn had been consumed (30 min later), the solution was analyzed by HPLC in the presence of KHP as described above, showing a 0.5% yield of benzoic acid (vs CrO?CI?). An identical Zn/HC1 reduction procedure on 0.36 mmol of basic chromium acetate [Cr,(O)(OAc), ..xH?OZ6] did not result in a detectable color change from the emerald green of the starting material. Kinetics of CrO2C12 Oxidation of Toluene and tert-Butyibenzene. Reaction solutions were prepared in the glovebox by addition of the appropriate volume of CrO?CI? to a solution of toluene (or de-toluene) diluted to 1.00 f 0.02 mL with cyclohexane in an 8 cm path length thick-walled Pyrex cell containing a stir bar. Addition of clear red CrOzCI? to toluene resulted in an immediate color change, forming a dark brownblack solution. Reaction solutions were degassed on the vacuum line while being cooled to -78 "C before the cell was mounted in the spectrometer. Data were collected and analyzed using the same techniques and apparatus employed above. Spectra were taken over the wavelength range from 300 to 700 nm using the cyclohexane solvent as a blank (vapor phase, at the appropriate temperature). Toluene concentrations were calculated assuming molar densities of the components in the solution are unchanged from the molar densities of the respective pure liquids at the appropriate temperature.?0 Reaction conditions and rate constants are reported in Table 2 . Addition of clear red CrO?C12 to terr-butylbenzene resulted in an immediate color change forming a dark brownlblack solution. Kinetic data were collected following the procedures used for cyclooctane. The plot of In[CrO?CI?]vs time is linear out to as long as data were collected (2.5 half-lives). The tert-butylbenzene concentration was taken from reported values for the molar density of the pure material.?O Reaction conditions and rate constants are reported in Table 2. (25) Balthis, J. H., Jr.: Bailar, J. C., Jr. Inorganic Synrheses; McGrawHill: New York, 1939; Vol. I. pp 122-124. (26)An old boale of "Chromium Acetate C ~ ( C ~ H ~ O ~ ) ~ -(Matheson, XH?O' Coleman & Bell) contained the basic acetate on the basis of its optical spectrum: Dubicki, L.: Martin, R. L. Ausr. J. Chem. 1969, 22. 701-7.
7146 J. Am. Chem. SOC.,Vol. 117, No. 27, 1995
t
I .o
"."
Cook and Mayer
, 0.00
0.04
0.08
0.12
0.16
0.20
ICrO2Cl2]mol&
Figure 2. Vapor-phase absorbance at 292 and 408 nm vs CrO2CI2 concentration in cyclohexane solution at 25 "C. Estimation of K,, for the CrO2C12 * CsHsC(CH3)3CT Complex. The 8 cm quartz cell was charged in the glovebox with CrO2C12, cyclohexane, and a stir bar and evacuated as described above. Vaporphase spectra were taken using the apparatus described above, against a neat cyclohexane blank (vapor phase, at 25.0 "C). The Cr02C12 absorbance at 292 and 408 nm in the vapor phase at 25.0 "Cwas found to be proportional to the concentration of CrO2Cll in the solution, as shown in the Beer's law plot of Figure 2 (at 25 "C the reaction of CrO?Cl? with cyclohexane was too slow to detect). From the slope of the plot, ~ 2 9 2 = , ~ 0.078(3) M-' cm-l and 6408nm = 0.049(2) M-' cm-' (these values relate the vapor-phase absorbance to the path length and the solution molarity and hence are not true extinction coefficients). Preparation and analysis of a solution of 0.123 M CrO?CI? in tertbutylbenzene at 25 "C in the same manner gives essentially the same values, EW,, = 0.078(3) M-I cm-' and E ~ O E ~ , ,= , 0.047(2) M-I cm-', so complexation of CrO?C12 by terr-butylbenzene must be quite weak: Kes 6 0.02 M-'.
Results Reactions of CrO2C12 with cyclooctane, isobutane, and toluene were carried out in the dark in sealed containers under vacuum in neat substrate or in cyclohexane solvent. Solutions were prepared in the glovebox and handled in the dark with the aid of photographic safe-lights. All reagents and solvents were vacuum transferred prior to use and cyclooctane, toluene, and cyclohexane were stringently purified prior to use (see Experimental Section). The,reactions all form brown precipitates, commonly referred to as Etard complexe~.~ Chlorinated products, such as benzyl chloride from oxidation of toluene, are detected in the solution, while other organic products are bound in the precipitate. Aldehyd? and ketone products are liberated on workup of the isolated Etard complex upon addition of a coordinating solvent such as acetonitrile or water. Alcohol products are only produced upon aqueous workup, usually done in conjunction with a mild reductant (KI or Na2S203). I. Oxidation of Cyclooctane by Cr02C12. A. Products. The reaction of Cr02Cl2 with neat cyclooctane over the temperature range 25-60 "C gives chlorocyclooctane, cyclooctanone, 2-chlorocyclooctanone, cyclooctanol, and cyclooctene oxide (eq 3, Table 1). Cr02C12 +
0-
The yields in eq 3 are from reactions at 40 "C and are reported as moles of product vs moles of CrO2C12 reacted; identification and quantitation was by GCmD and GCMS. Essentially the same yields are observed on reaction of CrO2C12 with a 4.35 M solution of cyclooctane in cyclohexane (54 mol % cyclooctane, Table 1). No cyclohexane oxidation products are observed, indicating that cyclooctane is substantially more reactive than cyclohexane, as is also evident from the kinetic studies described below. The oxidation of pure cyclohexane by CrO2C12 gives similar products (chlorocyclohexane, cyclohexanone, and 2-chlorocyclohexanone), although no epoxide and only a trace of cyclohexanol are ~ b s e r v e d . ~ The average oxidation state of the chromium in the isolated complex from the reaction of CrOzC12 with cyclooctane, determined by iodometric titration, varies from 3.94(3) for reactions run at 25 "C to 3.74(3) for reactions at 58 "C (Table 1). The presence of cyclohexane again has no effect. From the average final chromium oxidation state, the number of chromium oxidative equivalents expended in the reaction can be calculated. Comparing this value with the extent of oxidation of the organic products shows that the yield of observed cyclooctane products in oxidative equivalents is 66-68% (Table 1). There must therefore be other, unobserved oxidation products. The oxidative equivalent yield shows little if any variation with temperature or when the cyclooctane is diluted with cyclohexane, implying that the branch ratios leading to the observed and unobserved products are not very sensitive to temperature or cyclooctane concentration. A similar lack of mass balance in terms of oxidative equivalents in the oxidation of cyclohexane by CrO?C12 was explained by the formation of ring-opened compounds, primarily adipate {-O~C(CHZ)~CO?-}.~ Adipate was not observed but there was extensive indirect evidence for its formation. Adipate is formed by CrO2C12 oxidation of the cyclohexene intermediate. By analogy, the cyclooctane oxidations most likely form ringopened products, presumably octanedioate {-02C(CH2)&02-}, via the intermediacy of cyclooctene. While no cyclooctene is observed directly in these reactions (trace cyclohexene is observed in the reaction of cyclohexane), the detection of both 2-chlorocyclooctanone and cyclooctene oxide points directly to cyclooctene as an intermediate. Epoxides are well established as major products produced in reactions of alkenes with CrO2Cl2" and, in cyclohexane oxidation, it has been shown that the alkene intermediate is the only viable source of the a-chloro ketone product, 2-chlorocyclohexanone? Carboxylate products such as adipate and octanedioate are tightly bound to the resulting substitution inert Cr(1II) and are not liberated by the standard workup procedures (aqueous KI or aqueous Na2S203). Carboxylate-containing products can sometimes be displaced from the chromium after Zn/HCl reduction but even this procedure is not quantitative (see below). Reactions of cyclooctene with CrO2Cl2 support the contention that cyclooctene is an intermediate on the pathway to the unobserved products in the oxidation of cyclooctane. To model reaction conditions under which cyclooctene is formed and consumed in cyclooctane oxidations at 25 "C, aliquots of a dilute solution of cyclooctene in cyclohexane were added periodically to CrO2Cl2 in cyclohexane. For cyclooctene oxidations at 40 "C, a breakseal vial containing the cyclooctenekyclohexane (27) (a) Sharpless. K. B.; Teranishi. A. Y.: Backvall, J.-E. J. Am. Chem. Soc. 1977, 99, 3120-3128. (b) See also ref 5 and the discussion in Miyaura, N.: Kochi. J. K. J. Am. Chem. SOC. 1983,105, 2368-2378.
ci;+b+E+(-J+oo (3)
13.9%
19.6%
3.6%
3.6%
2.7%
(28) The k&s is not exactly proportional to [CgH16] as the second-order rate constant is I I % lower than expected based on the change in cyclooctane concentration. While this is larger than the uncertainty in the measurement, it may be due to small solvent effects or to the slightly different mix of products formed (Table 1).
J. Am. Chem. SOC., Vol. 117, No. 27, 1995 7147
C-H Bond Activation by Metal Oxo Species solution was broken into a CrOlCl?/cyclohexane solution within a sealed reaction vessel at the appropriate temperature. Substantially the same results are obtained at the two temperatures, with about half of the cyclooctene reacted being observed as chlorocyclooctanone and cyclohexene oxide (eqs 4 and 5). The
.2.0 7
\I
-3.0
0.16equiv
E+oo+b undetected + products
29(3)%
A Cro&
+
52(5)%
trace
19(4)Yo
(yields based on cycloctene) (4)
AnOc
(J..-'
0.12equiv
+
undetected products
(yields based on cycloctene) (5)
46(4)% 34(2)%
20(3)%
trace
ratios of 2-chlorocyclooctanone to cyclooctene oxide of 1 3 4 ) :1 at 25 "C and 1.7(3):1 at 40 "C are in good agreement with the ratios of 1.2(1):1 and 1.4(1):1 observed in the reactions of cyclooctane at the same temperatures (Table 1). This supports the conclusion that these products in the cyclooctane reaction derive from a cyclooctene intermediate. From eqs 4 and 5, roughly 50% of the cyclooctene reacted is oxidized to undetected product(s). The ratios of unobserved product(s) to 2-chlorocyclooctanone are 1.8(3):1 at 25 "C and 1.4(2):1 at 40 "C. From these branch ratios and the measured yields of 2-chlorocyclooctanone in the cyclooctani-reactions, we can calculate the yield of undetected products derived from cyclooctene in the cyclooctane oxidation. The cyclooctene pathway accounts for 5.4(8)% and 4.9(6)% yields of unobserved products at 25 and 40 "C. These yields are only slightly lower than the predicted 6.7% yields of octanedioate, calculated on mass balance considerations assuming that octanedioate is the sole unobserved product (Table 1). Thus a consistent picture of the reaction is derived (Scheme 1, see Discussion), accounting for 290% of the chromium oxidative equivalents consumed. B. Kinetics of Cyclooctane plus CrOzC12. Reactions of CrO2C12 with cyclooctane were followed by UV/vis spectroscopy, monitoring the disappearance of red CrO2C12 vapor from the headspace above the reaction mixture. The specific techniques and the special UV/vis cell employed (Figure 1) are discussed in the Experimental Section. There is a first-order dependence on [CrO2C12] under pseudo-first-order reaction conditions, in neat cyclooctane or in a solution of cyclooctane in cyclohexane. Plots of ln[CrO?Cl?] versus time are linear to approximately 4 half-lives (Figure 3). Varying the concentration of cyclooctane by addition of cyclohexane results in a proportional drop in the observed rate constant (/cobs) demonstrating the reaction to be first order in substrate as well (Table 2).28 This substrate dependence was assumed but could not be tested in the cyclohexane study.9 The activation parameters for the reaction of CrOZC12 with cyclooctane, from an Eyring plot of ln(W7') vs UT, are M = 19.1(2) kcal/mol and A,!? = -17.5(5) eu. These values are derived from the rate at which CrOZC12 is consumed. This is not the same as the rate of the initial activation of cyclooctane,
-7.0
I
0
loo0
2000
3000 Time (s)
4ooo
5000
Figure 3. A plot of In[CrOzClz] vs time for the reaction of CrO?Clz with cyclooctane at 50.0 "C.
because more than one CrOZC12 is consumed for each cyclooctane oxidized. (Cyclooctanone, 2-chlorocyclooctanone, and cyclooctene oxide each require '1 equiv of CrO2C12.) The linearity of the ln[CrO2Cl~]vs time plots indicates that the subsequent steps are much faster than the rate of cyclooctane activation and that the stoichiometry of the reaction does not change over time. Under these conditions, the observed rate of CrOZC12 disappearance is equal to the rate of initial activation times the reaction stoichiometry, the average number of CrO2Cl2 molecules consumed per cyclooctane activated. Values for the reaction stoichiometry vary very little with temperature or cyclooctane concentration (less than the estimated uncertainty, Table 1),29consistent with the observation above that the product branch ratios are not very sensitive to conditions. This supports the assumption of a constant reaction stoichiometry that was made in the study of cyclohexane oxidation by C I O ~ C ~ ~The .~O temperature dependence of the calculated rate constants for the initial step (Figure 4A)3' gives the activation parameters @ = 19.4(2) kcaYmol and A$ = -17.9(5) eu (Table 3). These are only slightly different from the values derived from the disappearance of CrO2Cl2 (see above). 11. Oxidation of Isobutane by CrO2C12. A. Products. Reactions of CrO2C12 with isobutane were carried out in cyclohexane solvent under pressure by condensing the isobutane into the reaction vessels from a gas addition bulb of known volume. A typical reaction was run at 60 "C in a solvent of 3.07 M isobutane and 5.69 M cyclohexane. Under these conditions, attack at cyclohexane is competitive with attack at isobutane. Calculations of the isobutane concentration in the cyclohexane solutions assume ideal behavior of isobutane and cyclohexane liquid and vapor phases (e.g., Raoult's law). Because of the pressurized reaction mixtures and the volatility of the isobutane oxidation products, detection and quantification required different procedures than were employed for reactions of cyclooctane or toluene. The volatiles were collected by shortpath vacuum transfer, yielding a cyclohexane/isobutane solution containing tert-butyl chloride, and chlorocyclohexane (quantified by GC/FID). The isolated Etard complex containing the bulk of the products was worked up by addition of dry dj-acetonitrile to displace aldehyde and ketone products which were then collected by short-path vacuum transfer. The remaining chromium-containing solid was hydrolyzed with Na~S203in D20, yielding alcohol products which vacuum transferred into the acetonitrile solution already collected. The CD3CND20 solution was analyzed by 'HNMR. Workup of a reaction of 0.0762 M CrO2C12 with 3.07 M isobutane in cydohexane (5.69 M) at 60 "C gives three major isobutane oxidation products-tert-butyl chloride {7.6(7)%), acetone {3.9(2)%}, and isobutyraldehyde {5.7(5)%}-plus a variety of minor products (eq 6, Table 4). The concurrent
7148 J. Am. Chem. Soc., Vol. 117, No. 27, I995
Cook and Mayer
1
of a carbon-carbon bond. This supports the proposal of carbon-carbon bond cleavage in the oxidations of cyclohexane and cyclooctane, for which there is only indirect evidence (see above). Acetone formation also indicates the production of a 1-carbon product. FTIR analysis of the volatiles stripped from the product mixture shows a small yield of CO?, 0.34(2)%. This is considerably less than the 3.9(2)% yield ,of acetone. The possibility that CO? could be bound in the Etard complex as carbonate was tested by acid workup of the precipitate (0.1 M HCl with Na&03), which gave only an additional 0.25% yield of CO?. Control experiments on solid K2CO3 gave quantitative recovery of CO?. The 1-carbon fragment unaccounted for apparently remains bound to Cr(III), presumably as formate. Isobutyraldehyde is a surprising product because it appears to derive from attack at a methyl group rather than the tertiary hydrogen. Similarly, CrO2C12 oxidation of 2-methylbutane gives 3-methyl-2-butanone (and 3-chlor0-3-methy1-2-butanone).’~~ However, the oxidation of t-dl -isobutane, (CH3)3CD, shows that isobutyraldehyde does not derive from initial attack at a primary C-H bond. The same products are observed from (CH&CD as from (CH3)3CH (Table 4), though in lower yields because reaction with cyclohexane is more competitive (see below). The isobutyraldehyde product observed by ‘HNMR shows a clean doublet for the isobutyraldehyde methyl protons-no deuterium is observed ut the tertiary position (eq 7). This indicates that
L
c
-18.5
3.00 IO’
3.10 IO’
3.20 lo’
3.30 IO’
3.40 10’
I/kmperature
-16.0
-16.5 .17.0
F
-17.5
-
-I8.0
c
.18.5 -19.0
-19.5 4
2.9 10’
3.2 IO’
3.1 10’
3.0 10’
Iiremperaiure
-12.5 -13.0
2 s
-13.5 -14.0
-
~.
